1. Technical Field
The present invention relates to the field of electrostatographic recording, such as electro-photography and electrography; and more particularly to an improved apparatus for monitoring the rate and amount of toner deposited by a development station.
2. Background Art
Generally, in electrostatographic image-forming machines such as printers and/or copiers, an electrostatic latent image is first formed on a member such as a photoconducting element. The electrostatic latent image is then developed into a visible image by bringing the latent image-bearing photoconductive element to a development station, whereat marking particles are deposited onto the electrostatic latent image. Specifically, in a xerographic process, the marking particles are in a dry format (i.e., they are not dispersed or suspended in a liquid medium).
So-called "two-component" developers consisting of pigmented marking articles and magnetic carrier particles are most commonly used in xerography. During the development process, the marking, or toner, particles, are mixed with the carrier particles and tribocharged against the carrier particles and become electrostatically attached to those carrier particles. The developer is then brought into contact with the latent image-bearing element using a development station, such as that consisting of an electrically biased metallic shell with a core consisting of a series of adjacent magnets. As the shell and/or core of the development station rotates against the electrostatic image-bearing member, the electrically charged toner particles are attracted to the electrostatic latent image and ultimately become attached to the photoconductive element. The carrier particles are electrostatically repelled by the electrostatic latent image and are electrostatically and magnetically attracted to the development station, wherein the resulting charge on the toner-depleted developer is dissipated and is mixed with fresh toner particles and made ready for subsequent imaging. Other types of developers also exist. For example, so-called "third-component developers" contain one or more particulate addenda added for charge stability, cleaning, or other reasons to a two-component developer.
Traditionally, the amount of toner deposited on the photoconductive element has been monitored using techniques such as on-line densitometry whereby the amount of toner in a certain test area of the photoconductive element is monitored. However, this technique cannot differentiate from factors within the development station from other factors which affect the amount of toner deposited in the test region. In addition, such methods only reveal how much toner is present after development, rather than determining the rate at which the toner is being deposited.
Recently, methods of monitoring the rate and amount of toner deposited within a development station have been disclosed in U.S. Pat. Nos. 5,006,897, 5,122,842, 5,235,388, and 5,285,243, the disclosures of which are specifically incorporated herein by reference. According to the techniques disclosed, a piezoelectric transducer is suspended into the xerographic developer. The transducer is first biased to attract the toner and the shift in resonance frequency associated with the mass of toner deposited at a given voltage and/or known bias potential, is determined. Subsequently, the sign of the bias is reversed and the toner particles are removed from the piezoelectric transducer, thereby readying it for the next measurement.
Although the use of a piezoelectric sensor does allow the toner mass deposited as a function of time and/or applied voltage to be determined and promises to be a very useful and powerful tool in xerography, the actual implementation of such a device into commercial equipment has been problematical.